This invention relates to novel diamic precursors for polymers, to novel polymers, and to novel reactions involving the precursors.
The invention is particularly concerned with the production of a novel oligomer which is useful in the production of imide foams. It is also concerned with novel imide foams and other products formed from the oligomer. It is also concerned with the processing of imide foams and their precursors, to provide greatly improved handling and mechanical properties of the materials.
Polyimide (imide) foams have been known for over thirty years. An early example of such foams is the reaction product of pyromellitic dianhydride (PMDA) with a polymethylene polyphenylisocyanate (PAPI). These foams have great heat resistance but they have not found general acceptance because they require a high cure temperature to produce the foam, they are physically weak and friable, they shrink when heated, and they are incompatible with fillers which are used in other foam systems to give the foam desirable qualities.
Many attempts have been made to improve the qualities and the ease of manufacture of imide foams.
For example, U.S. Patent 3,620,987 eliminates the need for external heat by reacting a polycarboxylic acid or a polycarboxylic anhydride with a polyisocyanate in the presence of a catalyst comprising a tertiary amine and an aliphatic alcohol containing one to six carbons. In U.S. Pat. No. 4,184,021, to Sawko, Riccitiello, and Hamermesh, the imide foam is formed by reacting PMDA with PAPI in the presence of a surfactant, a strong acid, and furfuryl alcohol to generate heat internally. These approaches require extreme care and produce a foam which is friable and lacks strength.
Several patents to Gagliani and associates (e.g., U.S. Pat. Nos. 4,439,381, 4,360,604, 4,305,796, 4,241,193, and 3,966,652) disclose imide foams which have superior physical properties, such as strength, flexibility, and ability to be filled. The physical properties are nonetheless not ideal, and such properties as flexibility are obtained only by using flammable additives. The foams also shrink when they are exposed to flame or thermal extremes, and therefore are not well adapted to protecting substrates from fire conditions. These foams, moreover, must be formed and cured at greatly elevated temperatures. Their use is therefore limited to relatively small products, and producing these products requires expensive and cumbersome equipment.
Presently known imide foams are also limited to relatively low density material unsuitable for structural use.
The cost of PMDA has also limited the wide application of imide foams.
In numerous other situations, fire- and heat-resistant polymers are required. For example, high temperature fibers, high temperature coatings, and binders for high temperature composites are widely sought.